Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's hemoglobin. A sensor is placed on a thin part of the patient's body, usually a fingertip or earlobe. Light of different wavelengths (red and infrared) is sequentially passed from one side to a photodiode on the other side. Changing absorbance of each of the two wavelengths is measured, allowing determination of the absorbances due to the pulsing arterial blood alone.
Traditionally, in a pulse-oximetry sensor, LED's are flashed at the same rate as the photodiode sampling rate. For a pulse-oximetry system to be able to remove indoor light artifacts, this rate needs to be at least 100/120 Hz, i.e. above the Nyquist frequency of the fundamental of indoor light (50/60 Hz), and more likely above 200/240 Hz given the first harmonic is often the dominant component. See, for example, “Artificial Lighting Interference on Free Space Photoelectric Systems”, Maximilian Hauske et al, EMO '09/Kyoto, 2009, paper 21 P3-1.
The acquired signal fully describes both wanted and unwanted indoor light information and allows for the separation of the two using digital filtering. At that flashing rate the LED's account for most of the power consumption of the pulse-oximetry sensor, which is in the order of 10 mW. If harmonics of the indoor light need to be removed as well (e.g. in case of fluorescent light, second and third harmonics can be significant), the sampling rate needs to be increased, with an equivalent effect on the power consumption.
As long as the power consumption of the sensor is not a critical parameter, the above-mentioned implementation is the simplest. An example of such an implementation is given in FIG. 1. This comprises a divide-by-2 counter receiving a clock signal 1. This is passed through an inverter to a pulse generator driving a red LED and directly to a pulse generator driving an infrared LED.
The signal is picked up by the photodetector, and passed through an amplifier to an analog-to-digital converter (ADC) driven by the clock signal through a delay line. The output of the ADC goes to a demultiplexer, which separates the red and IR components. The output of the demultiplexer is passed through a filter to remove the background illumination IL.
A timing chart for the system is shown in FIG. 2. In this figure, trace 1 shows the base clock of the system, traces 2 and 3 show how the Red and IR, respectively, LED timing is generated from the base clock, traces 4 and 5 show the Red and IR, respectively, LED drive pulses, trace 6 shows the sensed Red and IR signals on top of the unwanted indoor light signal, and trace 7 shows the sampling time base, in this case a delayed base clock, and the ADC triggers (vertical dotted lines) indicating where the samples are taken.
For a body worn wireless sensor or simple SpO2 monitor the power consumption of the LEDs in such a scheme is too large to be powered by a small battery such as a coin cell and still maintain an acceptable life-time.